(A) Illustration of HIPODSCO1. The assay is based on the UAS-GAL4 two-component system whereby a synthetic transcription factor mGAL4 expressed under the control of the ubiquitous RPS5a promoter activates the tdTOMATO-tagged SCO1 gene. These two components were combined with the sco1 mutant to generate pollen donor 1 and 2 (PD1 and PD2), respectively. (B) Pollen of PD1 and PD2 (blue, yellow) are applied to the stigma of a sco1 gynoecium (green). Gamete fusion involving two sperm from two different pollen donors leads to transactivation of the SCO1 gene resulting in dark green seeds and fluorescence of tdTOMATO in the embryo, while monospermy-derived seeds remain pale green with no fluorescence. (C) Silique and seed analysis of sco1 mutants containing only pRPS5a::mGAL4-VP16, (upper panel), only pUAS::SCO1-tdTOMATO (middle panel), and both pRPS5a::mGAL4-VP16 and pUAS::SCO1-tdTOMATO (lower panel). Scale bars, 500 μm and 100 μm in left and right panel, respectively.